This module abstracts out the process of choosing one of several
underlying implementations for a module. This can be used to provide XS
and pure Perl implementations of a module, or it could be used to load
an implementation for a given OS or any other case of needing to provide
multiple implementations.

This module is only useful when you know all the implementations ahead
of time. If you want to load arbitrary implementations then you probably
want something like a plugin system, not this module.
